Patent GB-1035729-A (GB)
Title: Improvements in or relating to optically accessible information stores
IPC: G11C13/04
Filed: 1964-11-30
Abstract: 1,035,729. Read-only stores. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 30, 1964 [Dec. 23, 1963], No. 48516/64. Heading G4A. An optical read-out store comprises a transparent medium wherein a series of light-reflecting layers are spaced at half-wavelength intervals in depth within each of a plurality of storage areas for each frequency to be stored in the area. Storing information (Figs. 5-7, not shown).- The light-reflecting layers are produced at the antinodes of a standing wave produced in the transparent medium by passing light of the required frequency through the medium and reflecting it back. The medium may be a photographic emulsion, diazo dye or alkali halide with colour centres. For each possible stored frequency, a separate monochromatic light source (laser or filtered arc lamp) is provided shining light through a polarizer followed by an information mask. The mask has a matrix of remanent birefringent elements, one for each storage area of the store, followed by an analyser crossed with the polarizer. To store the frequency in a selected area. the corresponding birefringent element is selected by energizing corresponding row and column wires of the matrix. Light from the analysers associated with the various frequencies is combined by means of beam splitters and imaged on to the storage medium using a size-reducing lens. Read-out.-In Fig. 9 (not shown), light containing all the storage frequencies is passed through a pair of birefringent crystals (which polarize it) and a quarter-wave plate into the store, those frequencies stored in any given storage area being reflected back to be diverted (at the gap between the two crystals) to pass through a series of light switches (one per possible frequency) each comprising an electrooptic phase plate followed by a pair of prisms. If the phase plate is energized, the resulting rotation of the plane of polarization causes the light to be deflected at the prisms through a corresponding filter which passes only one of the storage frequencies. If the phase plate is not energized, the light passes undeflected to the next of the series of light switches. Lightdeflecting means (Specification No. 976,610 is referred to in this connection) following each filter select one storage area and feed the light therefrom to a photodetector. By actuating the light switches in turn, all the frequencies stored in a given area are read from corresponding photo-detectors into a register. Fig. 11 (not shown) shows a modification in which a row of areas is selected at a time, optical fibres being used to feed a row of photo-detectors In addition, light switches are also used to select one of a number of stores. Fig. 12 (not shown) shows an embodiment in which a single frequency is used in the read-out light and a CRT is used to select a storage area by depositing charge on a corresponding area of an electrooptic crystal through which the reflected light passes to a single photo-detector Fig. 13 (not shown) shows an embodiment in which lightdeflecting means are used to direct light from a multi-frequency source to a selected storage area, the reflected light being passed through filters by means of beam-splitters to photodetectors, one per frequency.